An ATM switch generally comprises multiple input and output port adapters interconnected through a switching fabric. The main function of an ATM switch is to receive incoming ATM cells at the input ports and to redirect those cells to specific output ports for transmission into the surrounding network. Each cell includes a label or header containing routing information in the form of a Virtual Path Identifier (VPI) field and a Virtual Cell Identifier (VCI) field. The ATM switch reads these fields in each received cell and performs a table look-up operation in the input adapter in order to locate the target output adapter for the particular cell and to find new VPI/VCI values that are to be written (swapped) into the cell's label for use by the next ATM switch along the cells route.
In prior art ATM switches. the table look-up operations are performed only at the input adapters and all information obtained from the look-up operations, including the target output adapter identification and the new VPI/VCI values, must be transferred through the ATM switch to the appropriate output adapter. To reduce the amount of additional information that must be transferred through the switch, the new VPI/VCI values are written into the cell's label before the cell is transferred through the switching fabric to the target output adapter. The output adapter must rely on information received from the input adapter look-up operation in transferring the switched cell to the appropriate output port. An example of such a system is described in W. Fischer et al, "A Scalable ATM Switching System Architecture", IEEE Journal on Selected Areas in Communication, Vol. 9, No. 8, October 1991, pages 1299-1307.
The prior art has the drawback that dedicated processing of a cell with regard to a single connection is not possible on the output side of the ATM switch. The ability to provide such dedicated processing is desirable, however, to be able to support forwarding of ATM cells on the basis of assigned Quality of Service (QoS) properties. The term QoS refers generally to the fact that different cells may need to be treated differently; e.g., some cells may be given higher priority for transmission than other cells.